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354 STRUCTURAL AND FIELD GEOLOGY 
under the influence of gravity alone, escape to the surface. 
Or after penetrating to a great depth—far below the level, 
perhaps, of any valley in the neighbourhood—it may have 
its passage impeded or barred by the closeness of the joints. 
Subject to great hydrostatic pressure, it will now be forced 
to ascend to the surface along the same kinds of fissures by 
which it travelled downwards, and will issue as a deep-seated 
spring, which may or may not have a temperature exceeding 
that of the region where it appears. It is not improbable, 
indeed, that meteoric water may sometimes descend by 
fissures to such depths that its further progress downward 
is arrested by the internal heat of the earth. The enormous 
pressure at a depth of several thousand feet will prevent 
ebullition, but expansion must result, and this, added to the 
hydrostatic pressure of the descending currents, will force 

Fic. 130.—DRAINAGE IN MASSIVE IGNEOUS ROCK. 
the water through other fissures to the surface. Deep-seated 
springs of such a nature might cither be cold or thermal 
(Fig, 130), 
The underground drainage of schistose rocks is usually 
just as hard to determine as that of massive eruptives, 
Exceptionally, where schists are relatively well bedded and 
consist of a series of rocks, some of which are better water- 
bearers than others, springs will tend to appear at the out- 
crops of the latter. As a rule, however, owing to the 
abundant folding and the irregular jointing, the direction 
of the underground drainage among schistose rocks is quite 
indeterminate. 
A large number of strong springs often appear along the 
line of junction between an intrusive mass and the rocks 
it traverses, The water may be derived either from the one 
or the other, or from both, In Fig. 131, for example, a great 
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